1. Field of the Invention
The invention relates to a control apparatus for a four-wheel drive vehicle, a four-wheel drive vehicle, and a control method for a four-wheel drive vehicle that are capable of switching between a two-wheel drive state and a four-wheel drive state.
2. Description of the Related Art
A four-wheel drive vehicle capable of switching between a two-wheel drive state in which the vehicle travels by transmitting power only to main driving wheels and a four-wheel drive state in which the vehicle travels by transmitting power both to the main driving wheels and to auxiliary driving wheels is well known. Examples of such a vehicle include those described in Japanese Patent Application Publication No. 11-189065 (JP-A-11-189065), Japanese Patent Application Publication No. 11-157355 (JP-A-11-157355), and Japanese Patent Application Publication No. 11-157356 (JP-A-11-157356). The 4WD vehicles described in Japanese Patent Application Publication No. 11-189065 (JP-A-11-189065), Japanese Patent Application Publication No. 11-157355 (JP-A-11-157355), and Japanese Patent Application Publication No. 11-157356 (JP-A-11-157356), in particular, are constructed so that, for example, when the 2WD mode is selected, the auxiliary driving wheels that act as driven wheels during the four-wheel drive state are disconnected from a power transmission path to the auxiliary driving wheels, so that the rotary members of the power transmission path are stopped from rotating. That is, these vehicles are constructed so that during the 2WD mode, power is not transmitted to the power transmission path that is provided for the auxiliary driving wheels and, furthermore, the rotary members of the power transmission path are not rotated together with rotation of the auxiliary driving wheels that act as driven wheels during the two-wheel drive state.
However, in a construction in which there is a rotary member that stops rotating when the two-wheel drive state is entered, the rotary member and a member that contacts the rotary member are always in contact with each other at fixed contact portions during a stop of rotation. Therefore, there may arise a risk of decline in durability due to progress of local striking wear or the like that is caused by input of vibration from outside, for example, rotating vibration from an engine or the like. Concretely, the contact between a shaft and a bearing, the contact between a chain and a gear (sprocket), etc. are conceivable. For example, in the contact between the shaft and the bearing, the rollers of the bearing have line contact at fixed portions with a contact surface (rolling surface portion) of the shaft, so that there is possibility of striking wear (fretting) or the like being caused on the fixed contact portions by input of vibration from outside. In particular, in the case where the lubricating performance of oil has declined due to stop of rotation, this problem may more conspicuously manifest itself.
With respect to this problem, the foregoing patent application No. 11-189065 proposes that a link mechanism that links a rotary member that stops rotating when the 2WD mode is selected and a rotary member of a power transmission path to the main driving wheels is provided separately from a mechanism for the 4WD travel, and when the 2WD mode has been selected, the link mechanism is actuated on the basis of elapse of a predetermined time interval or the like so as to rotate the rotary member that stops rotating when the 2WD mode is selected, without switching the mode of drive to the 4WD mode. Besides, the foregoing patent application also proposes as still another related-art technology that, for example, in order to secure oil lubrication, the amount of oil is increased to increase the portion that is dipped into the oil, or the configuration of the ribs in the housing case is adjusted, or holes through which the oil is allowed to enter are provided, or oil is actively fed by using an oil pump, among other arrangements. However, there is possibility of drawbacks of cost increase or space efficiency deterioration related to the measure of providing the foregoing link mechanism separately from the mechanism for the 4WD travel and the like, efficiency deterioration due to a large amount of time needed for evaluation regarding various adjustments and the like, increased stirring resistance resulting from increased amount of oil, etc. These problems are not yet known to public, and no proposal has been made regarding a technology that restrains the contact at fixed portions and appropriately secures oil lubrication during the two-wheel drive state without involving drawbacks of cost increase, efficiency deterioration, etc.